31 December 2016

This is a tough review: two books about bridges which are written entirely in Dutch (I don't read Dutch).

I got this as a two-volume set, containing Bruggen in Nederland 1940-1950 (270pp) and Bruggen in Nederland 1950-2000 (367pp). Together, they form a sequel to the mammoth three-volume Bruggen in Nederland 1800-1940.

All these books are published by Nederlandse Bruggen Stichting (via publisher Walburg Pers), an independent "Bridge Foundation" set up to gather and disseminate knowledge about bridges, their design, construction and history.

Non-Dutch speaking readers may find these books difficult to get hold of. They are available directly from the publisher at a very low price (€39.95 for this two-volume set), but I think they don't ship outside the Netherlands. Otherwise, I'd recommend that anyone interested searches for them via Abebooks.

The 1940-1950 volume is a history of Dutch bridges in the Second World War and its immediate aftermath. An early chapter documents the preparation for a possible invasion, including the laying of explosives to allow bridges to be brought down quickly. When the Netherlands were invaded in May 1940, numerous bridges were destroyed. Some restoration work was undertaken while the country was occupied from 1940-1944, before a further phase of destruction took place as the country was liberated in 1944 and 1945.

All of this is documented at length in the book, with numerous historic photographs, including many of the bridges damaged and destroyed during the war. There are even aerial photographs taken during bombardment which show clearly how intense the effort was on both sides to deny opponents access to vital infrastructure.

The immediate post-war period is illustrated with numerous pictures of temporary bridges, of Bailey Bridge and similar construction, many of these massive in scale.

The 1950-2000 volume has 5 main chapters, covering spatial planning, urban planning and infrastructure; factors influencing bridge construction; developments in bridge construction; particular aspects of bridges; special bridges; and Dutch bridges abroad. The lengthy chapter on developments in bridge construction is subdivided to cover concrete, steel, moveable bridges, other materials, and substructures, just to give a flavour of the book's approach and coverage.

From what I can figure out, there is copious detail both on the context that Dutch bridges have been built within, and on the historical details of bridge design and construction technology within that context. It gives the definite impression that there is plenty of valuable information, pitched at an audience somewhere between the general public and the active professional.

As with the previous volume, the book is very well illustrated, with plenty of photos of specific bridges as well as explanatory diagrams and photos of specific bridge technologies and details. Most are in black-and-white but a good proportion are in colour.

It's difficult to comment on the text in either volume, but it appears to be thorough and well-researched. Both volumes have indexes and bibliographies, and the 1940-1950 book appears to have detailed cross-referencing to historical sources.

Together, they are a tremendously impressive pair of books, to an extent which just highlights the lack of similarly detailed appraisals of the bridges of other nations.

I had seen this bridge design presented at a conference, and came to the site expecting a level of disappointment. Elevations and photographs of the bridge indicated it to be somewhat cramped and truncated in its appearance. The mast seemed too short, cut off just above the cable stays rather than allowed to reach a polite conclusion higher in the air, and the 8m backspan seemed far too short to properly balance the 44m main span, whether structurally or visually. Awkward rectangular casings surrounding the bottom of each cable were also a real distraction.

It's worth noting that this was a design-and-build project, a process often noted for its ability to compromise quality in the pursuit of cheapness. The project had previously endured a stop-start history, with a number of designs proposed without ever getting anywhere.

In reality, the bridge was not only less awful than I had feared, but quite impressive in many ways. I got the feeling that here was a design team doing their best to eke what quality they could out of a tight budget and highly constrained site.

For the most part the bridge design is attractively simple, and I particularly like the parapets, which have a very clear and uncomplicated design, making good use of stainless steel handrails and infill mesh.

The front span of the deck is a plated box girder with outriggers supporting the main deck plate, while the back span is a plated box formed from a simple folded geometry. It is over-large compared to the front span, because it is so short that it needs to be deep enough to carry sufficient ballast to maintain even loads on the bridge's support. I'm guessing that land ownership restrictions are what led to it being so short.

The parapet mesh is also featured below deck level, presumably to prevent any attempts at climbing the bridge from underneath.

The mast is also in plated steel, although in a different manner. The masts of cable-stayed footbridges are normally hollow-sections, to provide sufficient strength and stiffness against twisting effects. Here, the mast comprises two steel plates arranged in the form of an open-tipped V, held apart by flat plates in a "ladder" arrangement. The bracing plates are inclined diagonally, which provides a modicum of torsional stiffness and prevents the main plates from buckling.

Flat plates exposed to wind often generate vortex shedding behaviour, which can cause vibration and even damage. The mast therefore incorporates a tuned mass damper half way up its height, hidden behind a cover plate, to prevent excessive vibration.

An interesting feature of the design is the use of colour. Most of the bridge is painted a very pale grey. Yellow is used to highlight the main elements of the bridge opening mechanism. A much darker grey is used for the bridges two approach ramps and western staircase, presumably to visually demarcate the main structure from ancillary elements.

The darker grey is also used on the box girder below the main deck span, including the various struts which support the deck plate. The effect is to put the structure below floor level in permanent "shadow", emphasising the thin fascia plate along the edge of the deck and making it look very slender. This is fine, but I don't think it works very well where it meets the back span: it emphasises the mis-match between the two, and leaves the front span looking far too spindly to be quite right.

I also can't admit to being a particular fan of the cable stay protection sleeves, which I assume are there to prevent vandalism. This is a difficult detail to treat successfully, and I've certainly seen far worse elsewhere, but if they are necessary, they are a necessary evil. Compare the similar detail on the Media City Footbridge, which has no anti-vandal shrouds but which elevates the level at which the cables connect to elements of the deck steelwork.

I do, however, love the timber benches which are situated below each of the cable shrouds. These are beautifully shaped and very well made. Each one is a different size, reflecting the different inclination of each cable that they sit below. They serve the dual function of providing a resting place and also of preventing people walking or cycling across the bridge from accidentally hitting their head on the cables.

I also very much like the various gates which are used to keep people off the deck while the bridge is being opened. There are four in total, each one detailed carefully according to its location. At the rear end of the bridge, there is a curved gate which slides into place below the approach steps, and a simpler swing gate to close off the ramp (also where the bridge operator stands to control the bridge movement).

At the front end of the bridge there are two swing gates which close off the steps and ramp, and when not in use, these are detailed to dovetail against each other, minimising the space they take up. It's a neat, very well-considered detail.

Who would have thought there would be so much to discuss on what is ultimately a relatively modest bridge? To be honest, there's quite a bit more which could be said, but I think this is enough. I've included a few more photographs to illustrate some of the other features of the bridge.

Perhaps my favourite has little do with the bridge's detailed design, but more to do with its situation at the mouth of the Deptford Creek. All along the south bank of the Thames in this area there are views across towards London's Docklands district, but there's something about the bridge which makes you stop and look afresh. It's a crossing point and also a vantage point.

05 December 2016

We're getting close to the end of the year now, which means this is probably a good time to remind any interested readers about deadlines for the Footbridge 2017 conference.

The conference will take place in Berlin next year from 6th to 8th September. The organisers are hoping to take the event in different directions to previous Footbridge conferences. As well as papers on innovation and dynamics, they are particularly looking for submissions which "tell a story" about a design or perhaps a new material or construction idea. They want contributors not just to share the facts, but explain their learning, to discuss not only their own work but enter into debate with others. It has the potential to be much more interesting and inspiring than a conventional conference.

They're also running an open request for design submissions, having identified six sites in Berlin where no footbridge is currently planned, but where one could be considered. It's not a competition as such, but a way to inspire creative contributions, which may be featured in a special book to commemorate the exercise, as well as discussed at the conference itself.

I'm particularly pleased to see that they plan to publish the conference papers freely online in April 2018, which I think is an excellent initiative.

The deadline to submit abstracts or to register to submit a design is 1st January 2017.

The viaduct is interesting for several reasons. The typical spans have semi-elliptical arch barrels each comprising four brick rings, but they vary in span, with span increasing towards the canal. There are then three main segmental spans, much longer at 87 ft span (26.5m), with arch barrels of bonded brickwork effectively the equivalent of ten brick rings thick.

A contemporary account from 1840 states that the more typical arches are of 30 ft span, "constructed of five rings, but shew only three on the face, which gives the entire structure a light appearance". This is obviously wrong, with four rings being visible, but it would be no surprise to find that the arch barrel below the tracks is thicker.

The 1840 account also explains why the bridge has cast iron rather than solid brick balustrades: "iron standards of the railing, very properly introduced instead of solid parapets, the ill effects of which are daily experienced on the Greenwich Railway: the noise to passengers is of a stunning description".

The main spans have stone piers and springing stones, as well as stone cornices and keystones, all topped with the attractive cast iron balustrade. However, the main thing that impresses me about this viaduct is the shear boldness and depth of the main arches. I’m no expert, but these look to have been very ambitious in brickwork at a time when many large span arches would still have been in stone masonry.

The bridge has lasted well, and following the closure of the railway in the 1960s, it was revived as part of the Docklands Light Railway. As can be seen in the photographs, some of the piers have been festooned with small pipework, which I assume to be an ad-hoc attempt to direct water seepage in the piers to a suitable drainage system.

It's a steel cable-stayed bridge, with a portal frame tower supporting the deck via an asymmetric fan-type cable layout. There are three pairs of main span cables, and one pair of back-stay cables. The bridge deck consists of steel edge girders supporting curved steel parapets and a timber plank deck.

At first sight, it's a slightly clunky but unobjectionable bridge, noticeable mainly for being awkward in height relative to the adjacent railway viaduct. There are two landings on the span which would have been better eliminated as they put ungainly kinks into the otherwise smoothly curving alignment of the main bridge steelwork.

Approaching more closely, two other features stood out for me.

The first is that the support arrangements are odd, with the deck bolted directly to steel supports at both the pylon and at the "tip" of the cable-stayed deck. With an asymmetric cable-stayed bridge, the normal arrangement is to fix the span at the position of the back-stays, and have an expansion joint and sliding bearings at the tip, so that the deck is loaded only in compression, and thermal expansion and contraction is catered for.

The second thing of note is that not only are all the support cables pretty slack, one has a pronounced kink in it, showing that it has failed inside a turnbuckle connection. Slack cables are not unusual on poorly designed short-span cable-stayed footbridges: it's often difficult to get sufficient weight in the bridge deck to ensure that all the cables remain tight. But it's unusual to find that none of the cables are tight, as this indicates that the entire deck is gaining no support from the cables.

On closer inspection, I spotted a half-joint in the main deck girders towards the tip end of the deck. This explained the fixed support at this end of the bridge, as any movement can take place in the half-joint instead. However, I was completely unprepared to then find a second half-joint on the other end of the span, closer to the mast, between the first and second pairs of stay cables.

This is a truly bizarre design. If the bridge deck has a genuine half-joint at both ends, then it presumably cannot transmit axial loads in either direction. However, a cable stayed bridge cannot function without axial loads in the deck, as these are necessary to balance the horizontal component of forces in the inclined cables.

The conclusion is therefore simple: none of the cables are in fact carrying any load from the deck, nor are they designed to do so. This accounts for their slackness. All four pairs of cables, and their supporting mast, are structurally redundant, just an affectation.

One positive thing about the slack cables is that this allows them to be threaded through the parapet railings in order to be connected directly to the deck stringers. Truly, this has to be one of the oddest arrangements I've seen on a bridge like this.

This is not the first time I've encountered "fake" cable-stay bridges (see, for example, this bridge in Salford). However, it is probably the most gratuitously stupid example that I've seen. There is, I think, sometimes a place for deceit in structural design – sometimes it may be necessary to give the visual impression of something rather than the reality because otherwise a structure may simply look wrong.

Here however, there is already a fine Victorian railway viaduct that can be admired – there was never any need for a short-span footbridge to try and compete for attention in this deeply ridiculous manner.